Not able to use another theme on my vista operating system

SOURCE: how to install vista themes??????

Go here and Download Windows VistaGlazz

Then
Go here and download the one you want and install it.

SOURCE: site for themes for windows vista operating system

http://www.askvg.com/new-aerovg-theme-for-windows-vista/Here is what you need!!! Click on the Above Link. Follow Each and Every Step!!!  
Hope This Helps!!!  Please Rate my Response!!! Thanks!!! 
roger

SOURCE: Can we download the Windows Vista theme on Windows?

• Download UX Theme MultiPatcher from here
• Run the setup file
• Grab anyone of the following styles and download them on to your PC
• Double click on the .msstyle file and apply the style.
• Voila! Your XP has a new look

The first two themes don’t require other third party software, but you need to install WindowBlinds to use the third theme.
Razor Vista(Download)

VistaVG(Download)

AeroGlass(Download)

Note that these visual styles can reduce performance, so go for them only if your computer has a good amount of resources.

from Windows Vista themes for Windows XP